BIOTECHNOLOGY – Vol. I - Cell Thermodynamics and Energy Metabolism - Horst W. Doelle
CELL THERMODYNAMICS AND ENERGY METABOLISM
Horst W. Doelle
MIRCEN-Biotechnology Brisbane and Pacific Regional Network, Brisbane, Australia
Keywords: thermodynamics, energy production, conservation, membrane, solute
transport, energy metabolism, enzyme catalysis
Contents
U
SA NE
M SC
PL O
E –
C EO
H
AP LS
TE S
R
S
1. Introduction
2. Concepts of Thermodynamics
2.1. First Law of Thermodynamics
2.2. Second Law of Thermodynamics
2.3. Free Energy
3. Concepts of Energy Production and Conservation
3.1. Principles of Electron Transfer and Transport
3.2. Proton-translocating Electron Transport Chain
3.3. Proton-translocating ATPase Complex
4. Concepts of Membrane and Solute Transport
4.1. Passive Diffusion
4.2. Facilitated Diffusion
4.3. Active Transport
4.4. Group Translocation
5. Concepts of Energy Metabolism
5.1. Photosynthesis
5.2. Aerobic Respiration
5.3. Anaerobic Respiration
5.4. Fermentation
6. Concept of Enzyme Catalysis
Glossary
Bibliography
Biographical Sketch
Summary
The most fundamental property of living cell systems is their ability to utilize and
transform energy involving thousands of individual and enzyme-catalyzed chemical
reactions. Since every chemical reaction involves a loss or gain of electrons, the amount
of energy released or used depends on the oxidation-reduction potential difference or
distance between the electron donor (oxidized compound) and electron acceptor
[reduced compound]. In order to maintain its integrity, gain and loss of energy must be
balanced via a controlled flow [electron transport] and energy transformation (ADP ↔
ATP), which follows the laws of thermodynamics.
On the basis of electron donor and electron acceptor availability, four modes of energy
production are recognized, namely photosynthesis, aerobic respiration, anaerobic
respiration and fermentation.
©Encyclopedia of Life Support Systems (EOLSS)
BIOTECHNOLOGY – Vol. I - Cell Thermodynamics and Energy Metabolism - Horst W. Doelle
The energy transformations are also vital for the transport of solutes along pH and
electrical gradients across the otherwise impermeable cellular membrane.
1. Introduction
One of the most fundamental properties of living cell systems is their ability to utilize
and transform energy. This energy occurs in a number of forms:
ƒ
ƒ
U
SA NE
M SC
PL O
E –
C EO
H
AP LS
TE S
R
S
ƒ
Mechanical Energy is developed during cellular movement, beating of flagella,
re-organization of intracellular structures such as mitochondria, and alteration of
cell shape;
Electrical Energy is produced when electrons move from one place to another,
usually expressed as a flow of current between two points due to a difference in
voltage;
Electromagnetic Energy occurs in the form of radiation, and in biology the most
significant is that from visible or near-visible light, such as radiation from the
sun for photosynthetic organisms. Some organisms release energy and glow,
which is referred to as bioluminescens. They produce light energy;
Chemical Energy is the energy that can be released from chemical reactions;
Thermal Energy or heat is produced as part of the normal energy transformation
processes and occurs as waste energy released into the surroundings;
Atomic Energy is contained within the structure of atoms themselves and is
released in the form of atomic radiation, which cannot be utilized by living
organisms.
ƒ
ƒ
ƒ
Since growth can be defined as the orderly increase of all chemical components, it is the
chemical form of energy, which is of greatest importance for the understanding of
microbial growth and metabolism.
2. Concepts of Thermodynamics
Cell metabolism consists of thousands of individual chemical and enzyme-catalyzed
chemical reactions. These chemical reactions in living organisms occur in
characteristically organized sequences, called metabolic pathways (see also Cell
metabolism). There are two main types of metabolic pathways:
ƒ
ƒ
Pathways which lead from large (low oxidative state) to smaller molecules (high
oxidative state), which are called catabolic pathways or catabolism
Pathways which lead from small (high oxidative state) to large molecules (low
oxidative state) essential for the formation of cellular material, which are
referred to as anabolic or biosynthetic pathways, or anabolism.
The main concept of catabolism is therefore to provide the cell with small molecules or
precursors suitable for biosynthesis of all major chemical constituents of the living cell
with reductant and energy to carry out these endergonic and reducing reactions leading
towards compounds of low oxidative state (Figure 1). Whereas all catabolic pathways
are oxidative and thus energy producing, the biosynthetic pathways are reductive and
energy consuming. Metabolism consists therefore entirely of energy transformation and
©Encyclopedia of Life Support Systems (EOLSS)
BIOTECHNOLOGY – Vol. I - Cell Thermodynamics and Energy Metabolism - Horst W. Doelle
transfer mechanisms, which are based on thermodynamics.
2.1. First Law of Thermodynamics
The amount of energy involved in a chemical reaction is expressed in terms of gain or
loss of energy during the reaction. The First Law of Thermodynamics is the law of
conservation of energy and states that the total amount of energy in nature is constant.
This means that, if heat [q] is added to a system of a given energy content, it must
appear as a change in the internal energy [ΔE] of the system or in the total work
performed by the system on the surrounding [w]
q = ΔE + w′
−
w′
(2)
U
SA NE
M SC
PL O
E –
C EO
H
AP LS
TE S
R
S
ΔE = q
(1)
Figure1. Generalized scheme for metabolic energy formation and usage
(adapted from Doelle 1994b).
Such an addition of heat results in many instances in a change of volume [ΔV] at a
constant pressure [P]
q = ΔE + PΔV
+ w′
©Encyclopedia of Life Support Systems (EOLSS)
(3)
BIOTECHNOLOGY – Vol. I - Cell Thermodynamics and Energy Metabolism - Horst W. Doelle
Whereby the expression ΔE + PΔV, representing the change of the heat content or
enthalpy, can be replaced by ΔH
ΔH = ΔE + PΔV
(4)
= q − w′
(5)
At any temperature, PΔV = nRT, where n represents the number of moles and R is the
gas constant [= 1.987 cal.moldegree−1] with T signifying the absolute temperature
ΔH = ΔE + nRT
(6)
U
SA NE
M SC
PL O
E –
C EO
H
AP LS
TE S
R
S
This expression reveals that each chemical reaction proceeds to completion with a
definite heat of reaction that is quantitatively related to the number of molecules
reacting. The energy unit for its measurement is the calorie, which is defined as the
quantity of heat energy necessary to raise the temperature of 1 g water by 1°C. One
calorie is equal to 4.184 joules.
Example:
C6H12O6 → 6 CO2 + 6 H2O
ΔH = −673 kcal = −2,815.8 kJ/mol
The negative sign indicates an exergonic reaction.
2.2. Second Law of Thermodynamics
Whereas the first law of thermodynamics implies only that there is a quantitative
correspondence between different kinds of energy and that the energy content in nature
is constant, the Second Law of Thermodynamics states that all physical and chemical
processes proceed in such a direction that the randomness or entropy of the universe
increases to the maximum possible, at which point there is an equilibrium. Since the
entropy has the symbol S,
ΔS = q / T
(7)
q = T ΔS
(8)
If one combines the equations of the first and second law of thermodynamics,
ΔH = T ΔS − w′
(9)
In considering these thermodynamic relationships one should always be aware that the
main interest in biological reactions is not in reactions at equilibrium, but in those
©Encyclopedia of Life Support Systems (EOLSS)
BIOTECHNOLOGY – Vol. I - Cell Thermodynamics and Energy Metabolism - Horst W. Doelle
proceeding in the direction that approaches equilibrium. The tendency to seek the
position of maximum entropy is the driving force of all processes, and heat is either
given up or absorbed by the surrounding system to allow the system plus its
surroundings to reach the state of maximum entropy.
These changes in heat and entropy are related by the free energy, ΔG
ΔG = ΔH − T ΔS
(10)
and since
ΔH = T ΔS − w′,
ΔG = − w′
U
SA NE
M SC
PL O
E –
C EO
H
AP LS
TE S
R
S
(11)
which expresses the energy released that is available to do useful work. The
determination of ΔG depends on accurate measurements of either the equilibrium
constant, K, of a reversible reaction or its electromotive force. The equilibrium constant
is very difficult to determine, since often no adequate analytical methods are available
to analyze the reactants and product concentrations.
2.3. Free Energy
Since metabolism consists of sequences of oxidations and reductions, the measurement
of the electromotive force is the choice for establishing ΔG.
This electromotive force is the algebraic difference of the potentials of two half-cells. In
order to understand this definition, it is necessary to realize that energy-yielding
reactions within the cell are of the nature of oxidations. An oxidation is generally
defined as the loss of electrons and reduction as the gain of electrons
H 2 + 2e− ↔
2H +
Electrons released by an oxidation MUST be accepted by an oxidizing agent, which
itself will be reduced. Such a transfer of electrons, according to modern theory,
establishes an electric current, since the electron donor possesses a characteristic
electron affinity. It should therefore be possible to obtain direct proof of the transfer of
electricity in oxidation-reduction reactions under suitable experimental conditions. This
transfer could be a quantitative measure of the tendency of substances to donate or
accept electrons and thus a means for calculating free energy changes for oxidationreduction reactions. This quantitative measure is termed an oxidation-reduction
potential.
The measured potential difference [Eh] of an oxidation-reduction system is expressed by
the NERNST equation
©Encyclopedia of Life Support Systems (EOLSS)
BIOTECHNOLOGY – Vol. I - Cell Thermodynamics and Energy Metabolism - Horst W. Doelle
Eh = E0 +
RT
nF
ln
[ Aox ]
[ Ared ]
(12)
where E0 is the standard electrode potential, R is the gas constant (= 8.314 J degree−1
mol−1), n represents the number of electron involved in the reaction, F is the Faraday
constant (= 96,494 coulomb) necessary to convert one equivalent of ions, and [Aox] and
[Ared] are the activities of the oxidized and reduced form of the oxidation-reduction
system.
U
SA NE
M SC
PL O
E –
C EO
H
AP LS
TE S
R
S
The standard electrode potential [E0] is the potential of an electrode in equilibrium with
a unity activity of its ions. This value is characteristic for each oxidation-reduction
system and gives a measure of the relative ability of that system to accept or donate
electrons in oxidation-reduction reactions.
The free energy change associated with an oxidative reaction may now be calculated
from the standard electrode potentials of the two reacting systems. For this, the
NERNST equation for the two systems has to be incorporated into the standard free
energy equation
ΔG = RT ln K
ΔE0 = ( RT / nF )ln K
nF ΔE0 = RT ln K
(13)
therefore
−ΔG = nF ΔE0
(14)
Let us demonstrate these calculations on an example:
malate + cyt.c →oxalacetate + cyt. C
malate/oxalacetate has a ΔE0 = - 0.17V
cyt. cox/cyt. cred has a ΔE0 = + 0.22V
therefore,
ΔG = − nF ΔE0
= −2 × 96,500 × [0.22 − (−0.17V )
= − 75.34 kJmol −1
Since electrons move only to a more positive redox system, the greater the difference
between the systems, the greater is the oxidizing ability of the system. Energy is
released in direct proportion to the difference in E0 values.
©Encyclopedia of Life Support Systems (EOLSS)
BIOTECHNOLOGY – Vol. I - Cell Thermodynamics and Energy Metabolism - Horst W. Doelle
-
TO ACCESS ALL THE 27 PAGES OF THIS CHAPTER,
Visit: http://www.eolss.net/Eolss-sampleAllChapter.aspx
Bibliography
U
SA NE
M SC
PL O
E –
C EO
H
AP LS
TE S
R
S
Bailey J. E. and Ollis D. F. (1986). Biochemical Engineering Fundamentals, pp. 228-305. New York:
McGraw-Hill Book Company. [This chapter 5 deals with thermodynamic principles, stoichiometry and
cell transport.]
Brock T. D., Smith D. W. and Madigan M. T. (1984). Biology of Microorganisms, (4th edition), PrenticeHall. [This book is an excellent introduction into microbiology.]
Broda E. (1975). The Evolution of the Bioenergetic Processes. Paris: Pergamon Press.
Carafoli E. and Scarpa A., eds. (1982). Transport ATPases. New York: New York Academy of Sciences.
Doelle H. W. (1975). Bacterial Metabolism. New York: Academic Press. [This book deals in details with
the metabolism of bacteria. Its pathways and individual enzyme-catalyzed reactions.]
Doelle H. W. (1990). Microbial Process Development. (Australian Academia of Sciences/Academia
Sinica/Unesco/MIRCEN Training Course). Beijing, China: Institute of Microbiology, Academia Sinica,
25 October to 17 November (1990).
Doelle H. W. (1991). The Importance of Microbiological Biotechnology for Community and Economic
Development. (Uneso/MIRCEN Regional Training Course) Papua New Guinea: Motupore Island,
University of Papua New Guinea, 2 to 7 September.
Doelle H. W. (1992a). Fermentation Technology, (African Regional Network for Microbiology & FADIB
International Workshop). Nigeria: Awka & Enugu, 7 to 19 September.
Doelle H. W. (1992b). Fermentation Technology for the Conservation of the Environment,
(Unesco/MIRCEN-Regional Training Course). China: Shanghai Institute of Industrial Microbiology,
Shanghai, PR China, 2 to 13 November.
Doelle H. W. (1994a). Microbial Process Development and the Ecological Environment in relation to the
Development of Fermentation Industries. (Unesco/MIRCEN/ICRO Regional Training Course). Vietnam:
Food Industries Research Institute, Hanoi 24 October to 2 November.
Doelle H.W. (1994b). Microbial Process Development. Singapore: World Scientific Publisher. [This
book emphasizes basic microbiological principles required for successful process development.]
Doelle H. W. (1998). Current Trends in Microbial Technology for a Sustainable Environment: Exploring
Microbial Diversity for Novel Processes. (Unesco/ICRO/MIRCEN/IOBB Regional Training Course).
Malaysia: University of Kuala Lumpur 12 to 14 October.
Klotz M. I. (1967). Energy changes in biochemical reactions. New York: Academic Press. [This book
emphasizes energy formation and usage through biochemical reactions.]
Kraemer R. and Sprenger G. (1998). Metabolism. Biotechnology Vol. 1 (eds. Rehm H. J. and Reed G.),
Biological Fundamentals (ed. Sahm H.), (2nd Edition). Germany: VCH Verlagsgesellschaft mbH,
Weinheim.
Kummerow F. A., Benga G. and Holmes R. P. (1983). Biomembranes and Cell Function. New York:
Academy of Sciences.
Morowitz H. J. (1970). Entropy for Biologists: An Introduction to Thermodynamics. New York:
Academic Press Inc.
©Encyclopedia of Life Support Systems (EOLSS)
BIOTECHNOLOGY – Vol. I - Cell Thermodynamics and Energy Metabolism - Horst W. Doelle
Nicolls D. G. (1982). Bioenergetics: An Introduction to the Chemiosmotic Theory. London: Academic
Press Inc.
Rosen B. P., ed. (1978). Bacterial Transport. Basel: Marcel Dekker Inc.
Biographical Sketch
U
SA NE
M SC
PL O
E –
C EO
H
AP LS
TE S
R
S
Horst W.Doelle, born in 1932, studied biology at the University of Jena (1950-1954). He studied for his
doctorate at University of Goettingen [1955-1957] on antibiotic production. After receiving his doctorate,
he worked in the Wine and brewing industry in Germany before taking up an appointment with CSIRO in
Australia in 1960. After 4 years wine research, he took up the challenge to build up microbial physiology
and fermentation technology at the Department of Microbiology at the University of Queensland in
Brisbane. He received his Doctor of Science in 1976 and his Doctor of Science honoris causa in 1998. He
perticipated and conducted numerous training courses in developing countries. After 29 years teaching he
retired in 1992. His research area was regulation of anaerobic/aerobic metabolism, microbial technology
(Zymomonas ethanol technology) and socioeconomic biotechnology using microorganisms for waste
management.
©Encyclopedia of Life Support Systems (EOLSS)